The present invention concerns a homogeneous nickel-based catalyst particularly for use in the hydrogenation of benzene.
The present invention also concerns a process for the production of cyclohexane by benzene hydrogenation, which is carried out in at least two steps. During the first step, benzene hydrogenation is effected in the liquid phase in the presence of a colloidal suspension of a nickel-based catalyst and at a pressure and temperature which enables a vapour phase to be recovered which contains at least a portion of the cyclohexane formed. During the second step, the vapour phase obtained in the first step is sent to a zone, termed the finishing hydrogenation zone, in which the products which can be hydrogenated contained in this vapour phase are hydrogenated in the presence of a solid catalyst.
Hydrogenation of benzene to cyclohexane is a highly exothermic reaction and is thus favoured by low temperatures and high partial pressures of hydrogen. A number of industrial processes have been developed and can be differentiated by their operational conditions, which depend on whether the reaction is carried out in the liquid or the vapour phase. These processes are also differentiated by the nature of the catalyst and the method used to compensate for the temperature rise due to the exothermicity of the reaction.
Among the most well known processes which use the liquid phase are the Hydrar process developed by Union Oil Product which uses a fixed platinum-based catalyst bed. In this process, the benzene feed and a cyclohexane recycle, mixed with fresh hydrogen or recycled hydrogen after recompression to the required pressure, are preheated then introduced into a series of two or three reactors in which the temperatures are stepped up between 200 .degree. C. and 300 .degree. C. and which operate at about 3 megapascals (MPa). Practically complete conversion per pass is achieved. The effluent from the reactors is cooled by heat exchange with the feed and a portion of the liquid is returned as the diluent to facilitate temperature control. The Houdry process uses three reactors in series operating with fixed beds with cyclohexane recycling. The Sinclair-Engelhard process uses a single reactor from which heat is eliminated in situ using a bank of tubes to produce vapour, and in which cyclohexane is not recycled. In the BP process, hydrogenation is carried out in two successive steps. In this process, the effluent from the first step contains about 95% by weight of cyclohexane and the reaction temperature is controlled by recycling liquid and vapour. In this process, temperature control using both the sensible heat energy and heat of evaporation of the recycled cyclohexane means that the recirculation ratio is substantially reduced, but the partial pressure of hydrogen is also reduced which means that a second, finishing, reactor must be used to complete the hydrogenation. Finally, in the Institut Francais du Petrole (IFP) process, the reaction is carried out in the liquid phase at a temperature of about 185.degree. C. at a pressure of 2 to 3.5 MPa in the presence of a nickel-based catalyst which is held in suspension by agitation using an external circuit. The hydrogenated product leaves the reactor in the vapour phase, helping to remove some of the heat. The rest of the heat given out by the reaction is recovered in an exchanger located in the external circuit (circulation of the stirring liquid phase) and used to produce low pressure vapour. The main drawback with this process is due to the fact that the catalyst used is a nickel which is pyrophoric to a greater or lesser extent, in which the colloidal particles are relatively large and which limits both activity and the stability of the suspension.
In order to use or sell cyclohexane today, a very pure product must be produced containing less than 500 ppm of total impurities. In particular, the majority of industrialised countries have imposed standards requiring the benzene content to be below about 100 ppm. These criteria mean that a relatively low temperature must be used (for example, less than about 300.degree. C.) along with a catalyst which does not favour either isomerisation of the cyclohexane to methylcyclopentane or hydrocracking. When a nickel-based catalyst is used, isomerisation only occurs above 250.degree. C.